Hi Atef -
I'm afraid that I haven't been able to look at your LEAP file. But it's entirely possible that the CO2 emissions constraint is being met, even without you explicitly applying the contraint. This would give you two scenarios with the same results.
You can check this by looking at the result displaying the emissions of the pollutant that you are attempting to constrain, in LEAP's results view. Verify that the total emissions sum for the optimization tranformation module, as well as all modules and final demands above it in the tree, is less than or equal to the constraint that you have applied. If this is the case, you may find that you need to lower your emissions constraint in order for it to have any effect on the model.
Hope this helps,
Taylor